In the non-contact measurement of the gauge of a high temperature material such as a red-hot steel slab, the high temperature material emits a thermal radiation of a wavelength corresponding to its temperature and therefore optical gauge measuring methods employing the ordinary visible light cannot be applied to the measurement of a gauge of such a high temperature material. In other words, the thermal radiation of a high temperature material (solid) initially lies in the infrared range and then, as the temperature rises, it produces a reddish light in the temperature range between 500.degree. - 600.degree. C. When the temperature rises further up to about 1,100.degree. C., the color of the light changes to yellow, and a white light is produced in the temperature range between 1,300.degree. - 1,500.degree. C. In this case, the thermal radiation from the material takes the form of a continuous spectrum corresponding to the respective temperatures of the material, and particularly at temperatures higher than about 700.degree. C. the wavelength band enters into the visible region of the spectrum. Consequently, if, for example, a high temperature material such as a red-hot steel slab heated to a temperature higher than 1,000.degree. C. in preparation for the hot rolling thereof is to be subjected to a gauge measuring process employing the ordinary light in the visible region, the temperature of the high temperature material to be measured is high enough to produce a thermal radiation that lies in the visible region as mentioned earlier. Therefore, a reference optical mark projected on the high temperature material will be hidden by the thermal radiation with the result that there is a tendency to cause an error in the measurement and moreover the effectuation of a measuring process itself may be made impossible.
Further, while a filter may be used in an attempt to prevent the transmission of the thermal radiation from a high temperature material, no satisfactory results are obtainable since the filter also acts simultaneously on the wavelengths of the optical mark itself.
In other words, it has been the practice in the art to distinguish an optical mark with the human eye or with such detecting means as a visible region vidicon and thus the wavelengths of the light used are confined to the visible region of the spectrum including the wavelengths between 4,000 to 7,000 A. Because of this fact, no matter in what manner a light source, a filter and a detector are combined, it has been impossible to completely eliminate the effect of the thermal radiation from a high temperature material and consequently it has been impossible to effect the required detection with good signal-to-noise ratio. For example, there are many different methods and apparatus for measuring the gauge of a high temperature material wherein, for example, a superhigh pressure mercury vapor lamp is used as a light source and an optical mark is detected with the human eye or a visible region vidicon through a band-pass filter having the maximum transmission sensitivity at 5,470 A or a light source comprising a superhigh pressure mercury vapor lamp is used and the detection of an optical mark is effected with the human eye or a visible region vidicon through a band-pass filter having the maximum transmission sensitivity at 6,480 A. However, such methods and apparatus are invariably affected seriously by the thermal radiation from a high temperature material and thus their accuracy of optical mark detection is invariably unsatisfactory.
Accordingly, in the measurement of the gauge of such a high temperature material, it is essential to use means for distinguishing the thermal radiation of the high temperature material lying in the visible region from an optical mark itself, and means for accurately converting, for example, the amount of change of position of the optical mark (i.e., the signal) in the thermal radiation (i.e., the noise) into a physical quantity, e.g., a mechanical displacement or an electrical quantity.